A wide variety of ionization sources, including atmospheric pressure ionization sources such as electrospray ionization or ESI, have been described for use with FAIMS. Unfortunately, ion separation in FAIMS is highly susceptible to contamination by neutral species and/or solvent vapor and liquid droplets. Accordingly, it is desirable to desolvate ions of an analyte species prior to introducing the ions into FAIMS, and to avoid introduction of neutral species into the FAIMS analyzer.
Typically, ions are produced at the ionization source and an electric field is established so as to direct the ions toward an ion inlet orifice into an analyzer region of FAIMS. Neutral species and droplets of the liquid solvent are prevented from entering the analyzer region by providing an outwardly directed flow of a desolvation gas. For instance, when a carrier gas is provided for transporting ions through the FAIMS analyzer region, often a portion of the carrier gas is made to pass outwardly through the ion inlet and perform the role of the desolvation gas. Alternatively, a separate curtain plate region is provided between the ionization source and the ion inlet into FAIMS. During use, a flow of a curtain gas provided into the curtain plate region splits into two separate flows; a first flow that is directed outward through an opening in the curtain plate and toward the ionization source, and a second flow through the ion inlet into the FAIMS analyzer region. Advantageously, the first flow desolvates the ions whilst the second flow assists in transporting the desolvated ions into the FAIMS analyzer region.
Of course, ions experience a variety of forces within the region between the ionization source and the FAIMS analyzer region. On the one hand, the force associated with the electric field pushes the ions in a direction generally toward the FAIMS analyzer region, whilst on the other hand the force associated with the outward flow of the desolvation gas pushes the ions in a direction generally away from the FAIMS analyzer region. In order to obtain high ion transmission efficiency into the FAIMS analyzer region, it is essential that these two opposing forces be well defined, and that the forces cooperate one with the other to direct the ions through the opening in the curtain plate or through the ion inlet directly into the FAIMS analyzer region.
Unfortunately, the flow of gas outward through either the opening in the curtain plate or the ion inlet tends not to be very close to perpendicular to the respective opening. In fact, the outward flow of gas has been found to be biased toward a side of the opening that is along the direction of gas flow within the region on the opposite side of the opening. This is because the gas has momentum, and tends to continue moving in the same direction as it passes outwardly through the openings. Accordingly, the flow of gas that is intended to desolvate the ions and prevent entry of neutral species also tends to deflect the ions away from the opening. If the deflection is of sufficient magnitude so as to cause some of the ions to impinge upon a surface of an electrode through which the opening is defined, then the efficiency of ion transmission into the FAIMS analyzer is reduced.
It would be advantageous to provide a system and method for introducing ions into a FAIMS analyzer that overcomes at least some of the above-mentioned limitations of the prior art.